The invention relates to a charge coupled imaging device with a semiconductor body in which a charge transport channel is defined at or adjacent a surface for the storage and transport of electric charge, which surface is provided with a series of gate electrodes in the form of conductive layers which are electrically insulated from the surface by an interposed gate dielectric, the device being designed for receiving electromagnetic radiation which is captured at least partly in or adjacent the charge transport channel through the gate electrodes and through the surface and is converted into electric charge through absorption.
The device may be a one-dimensional sensor or a line sensor. In the most widely used embodiment, the device forms a two-dimensional imaging device. A known type of two-dimensional imaging device which is used both in professional cameras and in consumer cameras is the frame transfer sensor, often called FT sensor for short, in which an image captured in the imaging section during a certain integration time is quickly transported into a memory section after the integration time in order to be read out there line by line. In the present document, the invention will be described in particular with reference to an FT sensor. It should be borne in mind, however, that the invention is not limited to imaging devices of the FT type, but may also be advantageously applied to other types of sensors such as line sensors or two-dimensional sensors in which the imaging section at the same time performs the function of a memory section, i.e. which do not comprise a separate memory section. In general, the invention is applicable to all types of CCD imaging devices in which the conversion of radiation into electric charge takes place in or at least in the immediate vicinity of the charge transport channel.
Since the charge transport layer in this type of imaging device at the same time forms the photosensitive elements in which the conversion from electromagnetic radiation to electric charge takes place, the radiation must penetrate the subjacent semiconductor body through the gate electrodes, at least in the usual case of illumination through the upper side. This always leads to a loss in sensitivity owing to absorption. The absorption is not the same for all wavelengths in the visible range of the spectrum, but comparatively great for blue and comparatively small for red when the gate electrodes, as is usual, are made of silicon, often in polycrystalline form, which is usually referred to as poly. Various measures have been proposed to prevent this loss of sensitivity. Thus it was proposed, for example, to illuminate the device from behind, where it is free from electrodes. This method, however, has the drawback that additional, complicated process steps are necessary, such as rendering the semiconductor body thin by etching or polishing. It was also suggested to use a different, more transparent material instead of poly such as, for example, doped tin oxide for the gate electrodes. Such materials, however, are usually less easy to handle and therefore less attractive in integrated circuits than is poly. It is further known to provide gate electrodes, whether or not made of poly, which have windows within which the surface of the semiconductor body is free from electrode material. This method does provide a considerable improvement and requires no process changes, but it has the drawback that the windows can only be comparatively small because a substantial portion of the channel must remain covered by gate electrodes.
It was suggested in the published European Patent Application EP-A 496,443 to use very thin poly for the gate electrodes instead of poly of the usual thickness (400-500 nm), for example, with a thickness of approximately 50 nm in order to limit the absorption in the poly as much as possible. Owing to the high sheet resistivity, i.e. the resistivity divided by the thickness, and the accompanying high resistance values in the poly tracks, it is not possible in this case to provide electrical connections at the edge of the matrix only, but the poly tracks are connected at regular intervals by means of low-ohmic tungsten connections which extend over the gate electrodes parallel to the transport direction of the device. The tungsten connections are separated from the poly electrodes by a dielectric layer in which windows are provided at the areas of the contacts. It was found in practice that the provision of these contacts on the thin poly may give rise to problems. It is possible in particular that, owing to an insufficient selectivity during etching of the contact windows, a too large portion of the poly is removed for a satisfactory contact to be formed. Another problem may arise with the use of a mosaic color filter in which sharply defined boundaries between the pixels corresponding to the mosaic pattern of the filter are desirable. When a single thin poly layer is used, the entire surface is in principle photosensitive, so that the boundaries between the pixels are not sharply defined. Furthermore, it is sometimes desirable to provide locally doped zones in the charge transport channel which must be accurately aligned in relation to the gate electrodes. Such zones are provided, for example, for the purpose of 2-phase operation such as described, for example, in U.S. Pat. No. 4,012,459. Another major application of such zones is to increase the charge storage capacity when the surface is brought into the inverted state during integration for reducing the dark current (All Gate Pinning or AGP mode), such as described inter alia in European Patent Application no. 93200598.6 filed Mar. 3 1993 under the title "Charge coupled device". The alignment of these zones relative to the electrodes is difficult in the case of a single poly layer.